Vauxhall Astra kangarooing

I have a T plate Astra which Kangaroo's when at cruising speeds. ie its ok when accelerating and decelerating but when cruising with the trafic flow it kangaroos. theres no loss of power and no warning lights. plugs and petrol filter changed when it first happened a few months back which seemed to fix it but problem is back again. have also put injector cleaner thru the system but no joy there either. only done a couple of thousand miles since changing filter and plugs so cant be that.. anyone any ideas?